Flexographic printing plates are produced by tightly fitting a negative film onto a photosensitive plate, applying an actinic ray thereto to expose part of the photosensitive layer of the plate to the ray, and washing and removing the unexposed portion of the photosensitive layer to form a relief. The solvent used for washing the unexposed portion is currently being converted from a conventional organic solvent into an aqueous solvent which is low in fear of environmental pollution.
The material for photosensitive layer of photosensitive rubber plate such as used in flexographic printing plate is required to have excellent transparency (which enables penetration of light to the interior of photosensitive layer), excellent balance between tight fittability onto material to be printed and strength of plate (the balance is hereinafter referred to as strength balance, in some cases) and high rate of washing with water (which enables easy formation of relief after light exposure).
As the material for photosensitive layer, there is known a photosensitive elastomer composition comprising a thermoplastic elastomer (e.g. styrene-butadiene block copolymer or styrene-isoprene block copolymer), a hydrophilic copolymer (these elastomer and copolymer are main components), a photopolymerizable ethylenically unsaturated monomer and a photopolymerization initiator.
Photosensitive rubber plates using the above photosensitive elastomer composition, however, are inferior in transparency and strength balance and low in rate of washing with water after light exposure.